In general, genetic assays typically examine a narrow portion of the information contained in DNA and RNA, the primary molecules of genetic information in living beings. It is increasingly becoming the case that a single genetic assay cannot provide the information needed for a technological objective, for example, blood typing, genetic profiling for drug responsiveness, characterization of stem cell pluripotency, etc. As such, using the information from several genetic assays to determine a final result is often challenging.
As mentioned above, there may be more than one genetic assay needed to perform blood typing, for example. The cells that make up the body's tissues and organs are covered with surface markers, or antigens. The blood group a person belongs to depends on the antigens on the red blood cells.
An antigen is any substance to which the immune system can respond. The antigens found on the body's own cells are known as “self-antigens.” These self-antigens are not normally attacked by the body's own immune system. For example, the millions of antigens contained in the membrane of each red blood cell are ignored by the body's own immune system. However, when patients receive blood transfusions of blood from other people, their immune systems will attack any donor red blood cells that contain antigens that differ from their self-antigens. As such, ensuring that the antigens of transfused red blood cells match those of the patient's red blood cells is essential for a safe blood transfusion.
As mentioned above, the antigens expressed on the red blood cell determine an individual's blood group. The main two blood groups are called ABO (with blood types A, B, AB, and O) and Rh (with Rh D-positive or Rh D-negative blood types).
Blood group antigens can be either sugars or proteins. For example, the antigens of the ABO blood group are sugars. In contrast, the antigens of the Rh blood group are proteins. The sugar antigens are produced by a series of reactions in which enzymes catalyze the transfer of sugar units. A person's DNA determines the type of enzymes they have, and, therefore, the type of sugar antigens that end up on their red blood cells. Similarly, a person's DNA holds the information for producing the protein antigens. The RhD gene encodes the D antigen, which is a large protein on the red blood cell membrane. Some people have a version of the gene that does not produce D antigen, and therefore the RhD protein is absent from their red blood cells.
Mixing of blood may occur during blood transfusions as well as during labor when a small amount of fetal blood may enter the mother's circulation. As such, it is important to be able to determine blood type to determine immune compatibility.